The present invention refers to a multi-cavity injection moulding system, which is adapted to be used for producing injection moulded parts consisting of at least two different plastic materials. More specifically, the present invention refers to a multi-cavity injection molding system comprising a back plate arrangement, a hot runner manifold arrangement with a first hot runner manifold for a first plastic material and a second hot runner manifold for at least one other plastic material, and injection nozzles fixed to the hot runner manifold arrangement for feeding the plastic materials to the relevant cavities defined in a cavity plate.
For many different cases of use, e.g. for plastic components used in motor vehicles, or as packaging material, or as shield means in electric or electronic devices, it is desirable to process a plurality of plastic materials of different kinds at the same time so as to obtain one plastic component; in these cases, a core layer will be embedded, and the plastic component will thus be provided with different material and/or surface properties and sandwichlike structures of materials will be obtained. For example, in the case of foodstuff packings, which are made of plastic material and which are to be used as longtime packings for highly perishable foodstuff or for foodstuff which has to be handled under difficult climatic conditions, it is necessary to embed in the first plastic material, which essentially defines the packaging body, a barrier layer of oxygen-impervious plastic. For this purpose, it is known to carry out in one injection moulding cylce joint injection moulding of two plastic melts consisting of different plastic materials (coinjection moulding, sandwich moulding) (cf. "Modern Plastics", February 1990, pages 54 to 56).
In the case of multi-cavity hot runner systems, it is, however, difficult to master the injection moulding process from the point of view of tool technology, and these difficulties prevented major progresses in the use of this multi-cavity injection moulding process for a prolonged period of time. It is especially difficult to avoid, by making use of a suitable control regime, a mixing of the various plastic melts outside of the moulding cavity and to form defined core films within an injection moulded part in connection with a basic layer of plastic material and a cover layer of plastic material within very short cycle periods.
It follows that hitherto known means for coinjection moulding (including sequential moulding) of different plastic melts very often include complicated injection moulding systems and control means, which, due to their degree of complexity, are susceptible to breakdown and are expensive, and the results achieved with the aid of these means were not always satisfactory.
For example, a multi-cavity injection moulding system for tri-injecting a plurality of thermoplastic materials to mould an article of multilayered structure is known from U.S. Pat. No. 4,808,101. Said apparatus includes a hot runner system comprising a first and second hot runner in a side by side arrangement resulting in a rheologically unbalanced flow of the different materials and a relatively complicated structure of the apparatus as, in this case, one of the three materials to be processed is fed from the side of the cavity of the apparatus while two melts are supplied from the backside as usual.
Similarly, EP-A 378 138 referring to a multilayer nozzle for an injection moulding system processing a plurality of different melts discloses a multi-cavity injection moulding apparatus wherein the different types of melts are supplied to the nozzles from opposite directions providing a lateral feeding of the nozzles.
DE-A 35 19 921 discloses a cavity injection moulding apparatus comprising a valve gated nozzle and a hot runner plate for feeding the two different types of melt to the nozzle wherein one plastic material is fed to the central melt bore receiving the valve pin while the other plastic material flows through a coaxial annular chamber towards the gate of the nozzle.